This invention relates generally to electromagnetic flowmeter systems, and more particularly to a secondary responsive to the electrode signal from the flowmeter primary, the secondary including a crystal-controlled feedback loop which functions to compensate for the adverse effect of fluctuations in the flowmeter drive current and other variables to provide a signal that accurately reflects the flow rate.
In an electromagnetic flowmeter system, the liquid whose flow rate is to be measured is conducted through a flow tube of a flowmeter primary provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an elecromagnet. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed in a converter or secondary to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Through A-C operation as disclosed, for example, in the Cushing U.S. Pat. No. 3,693,439 is clearly advantageous in that polarization is obviated and the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition (d.phi.)/dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In the U.S. Pat. No. to Mannherz et al., 3,783,687, whose entire disclosure is incorporated herein by reference, there is disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects.
While the signal yielded by the electrodes of the flowmeter primary is a function of the flow rate of the fluid being metered, it is also affected by changes in magnetic flux arising from fluctuations in the drive current for exciting the electromagnet and to flux changes produced by other factors such as temperature variations. To compensate for these variations, it is known to derive a reference voltage from the drive current and to divide the signal by the reference current to provide a ratio measurement.
In our prior art U.S. Pat. No. 4,167,871, entitled "Bi-Directional Electromagnetic Flowmeter," the signal from the flowmeter primary which is excited by a low-frequency wave is converted in a secondary into a variable-frequency pulses which exhibit a duty cycle proportional thereto, the pulses being sampled in a sampling circuit included in a feedback loop leading to one input of a summing junction. Also applied to this junction is the electrode signal, the output of the junction being fed to a demodulator. The sampling circuit acts to sample the reference voltage derived from the drive circuit of the flowmeter primary whereby the signal output of the secondary is compensated for variations in the excitation current in the primary.
The drawback to our prior arrangement is that no means are provided to stabilize the variable-frequency pulses; hence the effect of temperature variations and other variables in the secondary are not corrected.